1. Technical Field
This invention relates to apparatus and a method to control and guide telecommunications cables such as optical fibre cables, and is of particular relevance to the manner in which optical fibres are led into buildings at customer entry points, to connect the premises to the external trunk network.
2. Related Art
The telecommunications network at present comprises a trunk network substantially constituted of optical fibre, and local access networks constituted of copper wire pairs. It is expected that in the future the entire network, including the access network, will be constituted by fibre. It is known, however, that the properties of optical cable and copper wire are quite different, as discussed below, so that cable runs, building entry methods and network termination equipment for fibre and copper are different. This means that upgrading an installation from copper to optical fibre is expensive and cannot be easily achieved, as the infrastructure would generally have to be completely replaced.
The preferred method of installing optical fibre from the trunk network to customers' premises is by the blowing-in method (as originally described in EP 0108590). Blown fibre tubes are first fed along the outer wall of a customer's premises and then turned through 90 degrees, to pass through a bore made in the wall of the premises to enter therein. Optical fibre is subsequently to be blown into the tubes.
For the avoidance of doubt, the term “customers' premises” in this description includes all buildings into which telecommunications cables such as optical fibres are to enter. “Cable” shall be understood to refer to copper pairs, optical fibre, blown fibre, blown fibre tubes or other telecommunications cable, as the context permits.
A problem lies in one of the differences between copper pairs and optical fibre. Unlike copper, optical loss occurs at the sites of optical fibre bends or curves. Sharp bends can cause significant optical loss as well as mechanical failure. The tighter the bend, the worse the loss (source: John Crisp “Introduction to Fiber Optics” 2001 edition, page 53). Although fibre can be bent in use, excessive bending would result in fibre damage and/or significant optical losses. The limit beyond which the cable should not be bent as this would result in unacceptable radiation loss is known as the minimum bend radius (source: American National Standard for Telecommunications Telecom Glossary 2000).
A bare optical fibre comprising just the core, cladding and primary buffer has a minimum permissible bend radius typically of about 50 mm. A fibre cable including its outer protective layers, and bundles of optical fibre have even greater minimum permissible bend radius. Thus, even a single optical fibre cannot be bent directly through 90 degrees to go around corners. The need to respect the minimum bend radius requirement means that even a single fibre will protrude significantly at the site of the bend, e.g. where it enters an access bore in the wall of a customer's premises, and similarly will protrude as it exits the bore on the other side of the wall. (For ease of description, references henceforth to “entry” and the like of cables into bores shall include “exit” and vice versa, where the context permits.)
One known method to deal with this problem posed by the minimum permissible bend radius of the fibre as it exits the access bore, is to allow at least one loop of fibre to be fixed to the wall proximate to the access bore where the fibre exits. A relatively large customer lead in housing (CLI) unit is used to house the loop(s) of fibre, to protect it against interference and damage. The radius of the exiting fibre is generally not controlled within such a CLI unit.
An improvement which reduces the size of such a CLI housing unit is described in EP 0748460. This CLI unit is further described below against FIG. 1, but it essentially involves controlling the path of the exiting cable, to reduce the fibre's radius to substantially its minimum, while guarding against fibre breakage or loss.